1. Field of the Invention
This invention relates to a gas leak detection system for detecting leakage of gas from a gas supply path at points upstream of a gas meter.
2. Prior Art Statement
Recent years have seen a rise in the number of gas explosions occurring as a result of gas leaking from gas supply pipes. Underground gas pipes are apt to develop leaks owing to corrosion of the pipes, cracking as a result of uneven ground settlement and other causes. The risk of leakage is particularly high at hospitals, schools and the like, where gas is frequently supplied to the point of use from a remote storage facility through an underground pipe.
The method most commonly used for detecting leaks in such a gas supply path requires the installation of a pressure sensor in the gas supply path beforehand and involves periodic inspections conducted at regular intervals--annually or biennially, for example. The inspection is conducted by closing both ends of the supply path, raising the pressure inside the path to a prescribed level of, say 850 mm H.sub.2 O, and monitoring for pressure decrease, which, if found, indicates a leak.
Another method used is to continuously monitor the gas flow in the gas supply pipe using a flow meter. The detection of an abnormally large gas flow, i.e. a gas flow greater than the maximum expectable during ordinary gas consumption, indicates that a leak has occurred.
The former method involving periodic inspections is not generally capable of detecting leaks at an early stage because, for example, a leak arising shortly after the completion of one inspection will not be discovered until the next. If this should happen, or if a leak present at the time of inspection was too small to detect, a dangerously large amount of gas is apt to accumulate before the time of the next inspection. In addition, there is also a danger of the amount of leakage increasing with the passage of time owing to progressive corrosion of the pipe. In either case the risk of an explosion increases.
The latter method using a flow sensor to monitor the flow of gas through the pipe has a shortcoming in that the flow meter does not respond to leaks occurring upstream of the position at which it is installed. Though able to detect downstream leaks, it cannot detect those occurring upstream of the flow meter.
As already mentioned, at hospitals, schools and the like, the gas storage facility is frequently located far from the gas using equipment and the two sites are connected by an underground gas pipe. In such cases, the flow sensor is generally installed at the gas using end, either in a gas meter or on a wall of the building in which the gas using equipment is located. Since the flow meter is unable to detect a flow change limited to an upstream region of the gas supply path, there is a high possibility of it being impossible to detect a leak occurring in the underground gas pipe.
An object of this invention is to provide a gas leak detection system capable of simply and reliably detecting leakage of gas from the gas supply path at points upstream of a gas meter.